Nowadays, light-emitting diode (LED) as a backlight source of mobile phones, televisions, computers, and other electronic devices, are more popular. In general, the luminance of light emitted by the LED needs to be adjusted by manually or automatically follows the changes of the environment, as is known, LED is driven by a backlight driving circuit, and the backlight driving circuit changes a voltage output to the LED to change the luminance of the light emitted by the LED. As shown in FIG. 1, a common backlight driving circuit 1′ includes a LED driving chip 10′, a power port 20′, an inductor L, and a metal-oxide-semiconductor field-effect transistor (MOSFET) Q. The power port 20′ is used to receive a power voltage Vin. The LED driving chip 10′ is used to output a pulse-width modulation (PWM) signal to control the MOSFET Q to turn on or off periodically. Therein, when the MOSFET Q is turned on, the power voltage Vin is applied to two ends of the inductor L, the current flowing through the inductor L is enhanced, and the inductor L store energy; when the MOSFET Q is turned off, the two ends of the inductor L produce a backward inducted voltage connected with the power voltage Vin in series, thus providing a relative higher output voltage Vout to a LED module 2 together. Therein, Vout=Vin(1−D), D is a duty cycle of the PWM signal output by the LED driving chip 10′.
However, in general technique, the output voltage Vout is only determined by the duty cycle D of the PWM signal, when an amount of LEDs connected in series of the LED module 2′ is much more, the output voltage Vout is also needed more larger, and the duty cycle D is also needed to be a more larger value. When the value of the duty cycle D is larger, a duration that the MOSFET Q is turned on is very long, because the current flowing through the inductor is enlarged linear, then the current flowing through the MOSFET Q and the inductor L is larger, and the power loss of the whole circuit is large and the temperature of elements is high and the elements are easy to be damaged.